Optical metrology systems provide for inspecting properties of semiconductor wafers, where the wafers are positioned on a movable platform so that different areas on the wafer can be moved into position for inspection. The present invention relates to a prealigner used to determine, and in some cases adjust the position of the wafer on the platform.
This invention relates to optical metrology tools of the type described in U.S. Pat. No. 6,278,519 APPARATUS FOR ANALYZING MULTI-LAYER THIN FILM STACKS ON SEMICONDUCTORS which is incorporated herein by reference. These tools are used to analyze the characteristics of semiconductor wafers. Typically, these tools include one or more radiation probe beams which are focused to a small spot on the wafer surface, and include a motion stage for supporting the wafer. Various stage motion combinations are available including full X, Y stages; R/Theta stages; and xc2xd X-xc2xd Y plus Theta stages. The motions of the stages are computer controlled for moving the wafer into position with respect to the focused spot of the probe beam.
In the past, prior to loading a wafer onto the stage, the wafer was rotated to the desired orientation by a separate pre-aligner. This device would rotate the wafer and center the wafer to find the correct orientation for loading the wafer onto the stage. These pre-aligners were relatively complex and expensive.
Accordingly, the assignee herein developed a pre-aligner that was integrated into its metrology tools. The integrated pre-aligner mechanism is illustrated in FIG. 1.
As seen in FIG. 1, the wafer 10 is shown supported on a wafer chuck 12. The wafer chuck 12 sits on a rotating (theta) stage 16. The rotating stage 16 is placed on top of linear stages 6, 7, which can include full or half X-Y, or just a single linear stage. The theta stage 16 has an axis of rotation 17 and the wafer has a center axis 11. The wafer 10 sits below optics plate 18. Optics plate 18 supports optical elements (not shown) for focusing a probe beam of light onto the sample and for collecting the probe beam after reflection. As described in U.S. Pat. No. 6,278,519, measurements can include reflectometry and ellipsometry. The probe beam can be single wavelength or broadband.
Prior to measurement, the wafer 10 must be aligned with the optical elements of the measurement devices (not shown). A pre-aligner mechanism 22 is mounted on the lower surface of the optics plate 18. The mechanism includes a light source 26 in the form of a bar or elongated array of LED. A photodetector 28 is mounted opposite the light source 26. A gap 30 is defined between the light source 26 and the photodetector 28. The edge of the wafer 10 extends within this gap 30.
FIGS. 2A and 2B illustrate how the edge of the wafer 10 will overlap a portion of detector 28 when the wafer is in the gap 30. As can be appreciated, if the wafer 10 were loaded on the chuck in a position further left than the position shown in FIGS. 1, 2A and 2B, more of the photodetector would be covered and the voltage generated by the photodetector 28 would be lower. Conversely, if the wafer were loaded on the chuck 12 further to the right than the position shown in FIGS. 1, 2A and 2B, less of the photodetector would be covered and the voltage generated by the photodetector would be larger. This difference in photodetector output voltage corresponds to the position of the wafer can be used to determine the position of the wafer, and to align the position of the wafer.
In operation, after a wafer 10 has been placed on the chuck 12, the theta stage 16 is activated to rotate the wafer 10 through 360 degrees. The output of the photodetector 28 is monitored and stored as a function of rotation angle. Examples of the output from the detector as a function of rotation angle are shown in FIGS. 3A, 3B. As seen, there will be some point in the curve where there is a sharp discontinuity C2 in the measurement curve C1. This discontinuity C2 corresponds to the rotation angle when the wafer flat or notch 9 is aligned with the photodetector 28. The stage coordinates can then be determined relative to the flat or notch 9. FIGS. 3A and 3B illustrate that depending on the initial position of the notch 9 relative to the photodetector, the discontinuity will appear at different positions relative to the angle of rotation.
The output of the photodetector 28 can also be used to center the wafer 10. More specifically, if the wafer 10 was perfectly centered, the output voltage from the photodetector would not vary as the wafer was rotated and amplitude EX would be zero. If the wafer 10 is misaligned, the amount which the wafer 10 overlaps the detector 28 will vary as the wafer is rotated creating a sinusoidal variation in the photodetector output as shown in FIGS. 3A and 3B for example. The greater the variation in the sinusoid (from peak to trough (P1 to P2)), the more the wafer 10 is misaligned. By determining the extent of the variation (from peak to trough) and associating that with the wafer""s 10 angle of rotation, the amount of the wafer""s 10 misalignment can be determined. This information can be used to calculate a linear correction movement of the linear stages 6, 7 in correspondence with the orientation of the theta stage 16 such that the wafer""s center axis 11 may be used as reference during operational positioning of the wafer 10.
In the past, the above described pre-aligner has been used successfully in stand alone metrology tools. Some difficulties have arisen when attempting to use this approach with integrated metrology systems, or those measurement systems incorporated into semiconductor fabrication tools. In such systems, it is sometimes difficult to control ambient light falling on the detector. This is particularly true if the distance between the light source and the photodetector is increased as was found to be necessary to accommodate alternate wafer load mechanisms used in an integrated metrology environment. Changes in ambient light can adversely effect the operation of the prior art system. Particularly fluorescent illumination with its well-known flickering characteristic imposes a significant ambient distortion. Since changes in ambient are less controllable, it would be desirable to develop an improved wafer alignment system that could operate where ambient light variations exist.
The present invention is directed to a system and a method for determining the alignment of a wafer. The invention uses a light source which is modulated, and utilizes synchronous detection of the modulated light generated by the light source. One embodiment of the invention is a system for determining the alignment of a wafer supported by a chuck coupled to a stage capable of rotary motion. This system includes a light source, and a photodetector positioned a distance from the light source, wherein a gap is defined in the distance between photodetector and the light source. The wafer is positioned such that a portion of the wafer is disposed in the gap. The photodetector generates an output in response to light received from the light source. In this system a first circuit is connected to the light source and to the photodetector, and provides modulated power to the light source. The circuit also provides synchronous detection of the output of the photodetector and generates signals based on the output of the photodetector. This system also includes a processor for interpreting the signals generated by the photodector to evaluate the alignment of the wafer.
Another embodiment of the invention includes a method for determining the alignment of a wafer supported by a chuck coupled to a stage capable of rotary motion. This method includes providing a modulated light source, where the light source is modulated at a first frequency, and providing a photodetector which outputs a first signal in response to modulated light generated by the modulated light source and in response to ambient light. This first signal is processed to generate a second signal which corresponds to the modulated light, and the second signal is analyzed to determine the alignment of the wafer.
Another embodiment of the invention includes a method for determining the alignment of a wafer supported by a chuck coupled to a stage capable of rotary motion. This method includes driving a light source with a modulated power supply, whereby a modulated light is emitted into a gap. A photodector outputs a first signal in response to modulated light generated by the light source and an ambient light. This signal is processed to generate a second signal. A wafer is positioned such that a portion of the wafer is disposed in a gap between the light source and the photodetector, and the wafer is rotated. Changes in second signal that result from rotating the wafer are then analyzed to determine the alignment of the wafer.